This invention relates to refining of ore containing aluminum compounds. More particularly, this invention relates to improving the yield in a Bayer process by processing of the spent liquor.
Ores containing aluminum compounds, more particularly bauxite type ores, are conventionally digested in caustic by the well-known Bayer Process to form a solution of sodium aluminate and sodium hydroxide. The solid bauxite residue (red mud) is then separated from the solution by clarification and, after washing to remove as much caustic content as economically feasible, is discarded. Alumina is recovered by precipitation from this solution by first cooling the solution to supersaturation and then seeding with aluminum trihydroxide. The spent mother liquor is then recycled back to the digester, after an evaporation step, for reuse of the caustic therein and to preserve the alumina values remaining in the spent liquor after precipitation.
Unfortunately, however, such ores may contain or form many other soluble compounds such as sulfates, fluorides and carbonates which, due to their solubility, are not removed in the clarification step. These unwanted impurities can build up in concentration with repeated recycling of the spent liquor which can retard the recovery of alumina from solution, deposit scale on equipment (particularly heaters), reduce evaporation efficiencies, increase the required plant size (for given throughput) and operating costs due to the circulating liquor loads and increase the impurity level in the product. It would, therefore, be desirable to efficiently separate out such impurities from the Bayer liquor.
The yield of alumina in the precipitation step is further influenced by the ratio of alumina to caustic in the liquor. As the caustic level rises, due to depletion of alumina by precipitation, further precipitation of alumina is inhibited. This is sometimes overcome by neutralization of the sodium hydroxide, e.g. with sodium bicarbonate. However, this requires further reprocessing of the neutralized liquor to recover the soda for reuse in the digester.
If the caustic in the spent liquor could be selectively removed after the initial precipitation of alumina, the spent liquor stream could be reseeded and further alumina values could be precipitated from the spent liquor.